Thermal spraying is a process of melting and propelling fine particles of molten material such as metal to form a coating. One or two wires or a powder may be used for feed material, and heating is by an electrical arc or a combustion flame. One type of thermal spray gun is a dual wire, arc thermal spray gun, in which two wires are fed into contact at the wire tips that are melted by an electrical arc with current passed through the wires. A jet of compressed gas (usually air) is blown through the tips to atomize (i.e. nebulize) the molten metal and effect a spray stream of molten metal particles. Arc current generally is of the order of hundreds of amperes. A variety of gas head configurations have been suggested, for example as disclosed in U.S. Pat. Nos. 3,546,415, 4,095,081, 4,492,337, 4,668,852, 5,714,205, and 5,791,560.
Some applications involve coating inside surfaces of holes or other confined areas such as cylinder bores. For such applications an extension gun is used in which a gas jet from the side deflects the spray at an angle from the main axis so that the gun can be inserted into the hole with the angled or deflected spray directed to the surface. The side jet may be auxiliary to a central atomizing jet and may or may not serve as the primary atomizing jet. For spraying an inside surface of a hole, such as a cylinder bore of an internal combustion engine, either the gun must be rotated on its axis or the object with the hole must be rotated at a fairly high speed while the gun is moved axially. Such rotation of an object such as a cylinder block is generally impractical. Single wire combustion gun extensions or powder guns can be rotated with appropriate mechanisms. A single wire arc gun with a rotating non-consumable electrode is disclosed in U.S. Pat. No. 5,245,153, but single wire arc guns have not become commercially viable, at least in part because such electrodes are really not "non-consumable" at the high currents.
Angular two wire arc guns are disclosed in U.S. Pat. No. 4,853,513 and in patent application Ser. No, 09/038,435, filed Mar. 11, 1998 of the present inventor and assignee. For a two wire gun, a complex system is necessary to rotate the spray head about the wires to avoid twisting. Irregular spraying can be expected from the varying geometry with respect to the converging wires. Spraying with a two wire arc gun is relatively cheap and, therefore, desirable for many applications.
Accordingly, an object of the invention is to provide an improved, dual wire, extension type of arc thermal spray apparatus for spraying inside of a hole in an object without need for rotating the spray apparatus or the object. Another object is to provide such an apparatus that effects a conical fan spray for spraying inside of holes. A further object is to provide a novel gas cap for such an apparatus in order to achieve the foregoing objects.